Noroxomaritidine reduction by NR. Enzymatic activity of NR for reduction of noroxomaritidine (m/z 274.3) was monitored using LC-MS/MS with the same LC time program found in Ref. 18. A, enzyme assays of NR with 100 mm sodium phosphate buffer, pH 7.0, 100 μm noroxomaritidine, 1 mm NADPH, and 10 μg of pure protein in 100 μl at 30 °C for 2 h. Traces from top to bottom are: noroxomaritidine standard; complete assay with NR, noroxomaritidine, and NADPH; assay without NADPH; assay without noroxomaritidine; and assay with TALON resin purified E. coli empty vector protein extract substituted for NR protein. B, EPI MS/MS of NR product. C, EPI MS/MS of noroxomaritidine reduced with NaBH4.